This invention relates to the casting of metal strip. It has particular but not exclusive application to casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
Twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, for example aluminium. However, there have been problems in applying the technique to the casting of ferrous metals. As a consequence of the much slower rate of solidification of ferrous metals, it is absolutely critical to achieve an even cooling and solidification at the casting surfaces to allow continuous casting to proceed satisfactorily. This can be very difficult to achieve, particularly at the commencement of a casting run. Generally it requires that the molten metal be caused to flow through small flow passages formed in refractory material in a metal delivery nozzle. Although the metal delivery nozzle is preheated prior to a casting run, the refractory material around the small flow passages is very prone to localised cooling which can lead to premature solidification of the molten metal, particularly during start-up. It has therefore been necessary to supply the molten metal to the delivery nozzle at temperatures well in excess of the liquidus temperature of the molten metal in order to ensure that none of the metal solidifies prematurely due to localised cooling effects as it passes through the delivery nozzle. Typically, the metal may need at start-up to be preheated so as to have more than 100.degree. C. superheat, i.e. to a temperature more than 100.degree. C. above the liquidus temperature of the metal. Prior to the present invention, in the case of low carbon steels which have relatively high liquidus temperatures, to achieve this and to compensate for heat loss not only at start-up but for the duration of the casting run, the temperature of the molten metal charge upon tapping from the furnace may need to be over 1700.degree. C.
The heating of large quantities of molten metal to temperatures of the above order requires considerable consumption of energy and it also presents obvious problems for operational safety, as well as dramatically curtailing the effective life of the casting rolls and refractory materials, all of the above having a significant effect on operating costs. We have determined that after the initial start-up phase of a casting run, the refractory material of the delivery nozzle is raised uniformly in temperature through heat transfer from the molten metal so that the extremely high temperatures of molten metal are thereafter not necessary to prevent premature solidification. Moreover, we have further determined that such high temperatures also drastically restrict the productivity of the caster in that higher solidification rates can be achieved if the temperature of the casting pool can be lowered.
It has previously been proposed to minimise the superheat of the molten metal in continuous slab casters by supplying supplementary heating to the metal as it flows through a tundish and immersion nozzle to a continuous casting mould in order to prevent premature solidification. U.S. Pat. No. 4,645,534 of D'Angelo et al describes heating of the flowing metal by passing electric current through it from a heating device such as a plasma torch. The heating device may be applied to the metal in the tundish and the current caused to pass through the flowing metal to the immersion nozzle or the mould downstream from the tundish. Japanese Patent J91018979-B (Publication No J59202142) also discloses heating of metal as it flows from a tundish through an immersion nozzle into a continuous casting mould by passing electricity from a plasma torch in the tundish to an anode connected to the immersion nozzle.
The proposals described in U.S. Pat. No. 4,645,534 and Japanese Patent J91018979-B are not directly applicable to twin roll casting of thin strip. The problem of premature solidification in the multiplicity of small flow passages of the delivery system on start-up cannot be overcome by the supply of instant heat during a casting run, because it is not possible to transfer energy into the metal at a sufficient rate or to control the transfer to a degree sufficient to maintain temperatures and therefore flow rates through the flow passages of the delivery systems. The present invention addresses the problem in a different manner by providing a method and apparatus whereby molten metal can be supplied to the delivery nozzle at a relatively high temperature on start-up but at a significantly lower temperature throughout the remainder of a casting run.